​I'm running into an issue when I tried to build this: http://www.instructables.com/id/20-minutes-USB-solar-charger/. I'm using a 9v solar panel. As soon as I connect the ground from the lm7805 to the ground of the circuit, my multimeter reads 0 volts on the output pin on the lm7805. Before I connect the ground, it reads about 9 volts. I even replaced the lm7805 thinking something was wrong with it. I think I'm not understanding something simple, but I don't know what it is. I'd appreciate some insight into this issue I'm having.

One possibility I'm wondering about is if the lm7805 requires a minimum current, and the solar panel isn't meeting some minimum current requirement. Is that even a possibility?

  • \$\begingroup\$ If it generates 9V when no load is attached that is the minimum load (0 amps). I suspect there is a short circuit to ground in the powered circuit that brings the output to 0V. \$\endgroup\$ Commented Nov 14, 2015 at 20:02
  • \$\begingroup\$ A photograph of your setup would be helpful, anyway. But be sure you check all the connections in your circuit. \$\endgroup\$ Commented Nov 14, 2015 at 20:03
  • \$\begingroup\$ BTW, the output of the 7805 should give 5V, not 9V. If you connected the voltmeter correctly then you probably fried your IC, or you exchanged the input for the output. \$\endgroup\$ Commented Nov 14, 2015 at 20:06
  • \$\begingroup\$ Does your circuit work with, say, a 9V battery? \$\endgroup\$
    – GPS
    Commented Nov 14, 2015 at 20:12

1 Answer 1


An LM7805 does not require a minimum load, however it draws a few mA itself, even with no load. If the output voltage drops to near zero with just the regulator, that implies that the solar cell is getting very little light (in a dim room), is too small or is defective.

  • 1
    \$\begingroup\$ OR the regulator itself is mis-wired or defective (oh WHY did they swap the pins around on the 78M05?????) \$\endgroup\$
    – user16324
    Commented Nov 14, 2015 at 21:10

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